[evlatests] Source position offsets versus distance from calibrator

[Ed Fomalont]

     COMMENTS ON SOURCE POSITIONS FROM C0555plus_27sep09 DATA SET

     October 2, 2009    Ed Fomalont


EXPERIMENT DESCRIPTION:

    Phase referencing between 0555+398 with 5 other calibrators within 
16 degrees.
    4.68 GHz, with 4 IF's, each with 256 channels of 0.5 MHz each (the 
usual)

EXPERIMENT REDUCTION:

    Used 0555+398 as phase reference.  Imaged other sources
    Analysis only of source positions.  Assumed that observed
      phase center was the true source position.

QUICK SUMMARY:  All source displacements are directly proportional
    to their distance from the phase calibrator.  But, the source
    position dependence over the frequency band varies INVERSELY with
    frequency!


COMPARISON OF SOURCE POSITION OFFSET VERSUS SOURCE SEPARATION
                     From 0555+398 

SOURCE    FLUX     DISTANCE      POSITION       RATIO OF
                  FROM 0555       OFFSET      POSITION OFFSET
                  Sep  angle    Sep  angle         TO
           (Jy)  (deg)  (deg)  (asec) (deg   DISTANCE FROM 0555

0555+398   5.50     0.0   00     0.00   00        --
0559+238   0.62   15.93  177     4.28  174      0.0000746
0604+442   0.55    4.71   21     1.29   21      0.0000761
0625+446   0.18    7.34   48     2.03   52      0.0000768
0644+392   0.59    9.53   93     2.73   97      0.0000796
0542+498   8.4    10.30  -13     2.85  -22      0.0000769

FREQUENCY BEHAVIOR

0542+498 ALL      10.30  -13     2.85  -22      0.0000769

0542+498 IF1                     2.98  -23      0.0000804  
0542+498 IF2                     2.93  -24      0.0000790  
0542+498 IF3                     2.84  -22      0.0000765
0542+498 IF4                     2.75  -24      0.0000742

0542+498 IF1 chan   1-128        2.97  -22      0.0000800 
0542+498 IF1 chan 129-256        2.93  -23      0.0000790 

   i.e. source position varies with inverse frequency observed!!

   Time variability is hard to obtain.  Some indication the the offset 
ratio decreased by 10% over
     the five hours.


CONCLUSIONS:

 o  Source position offset varies directly with distance from phase 
calibrator

 o  Source Position offset / Source Distance from Calibrator  = 0.000078 
+/- (02)
    for this experiment (all sources)

 o  By the way, this ratio corresponds to a delay error of 4cm,
    similar to Vivek's 0212 analysis reported yesterday

 o  BUT, different IF's have different position scaling,
        varying inversely proportional to the IF, channel frequencies!!

 o  The progressive deterioration of source images with distance from 
the calibrator is
    explained by the source position change over the observed bandwidth.

 o  The above properties do not behave like a simple frequency scaling.  
For example
    a frequency offset of

            4.8 GHz * 0.000078 = 374 KHz +/- 10 KHz

    would explain the average source position change, but not the 
position variation over the
    observed bandwidth.  A simple delay error also does not explain this.

 o  Position analysis of the 0212 data may suggest how variable this 
effect is

 o  Similar experiments, but cycling among difference frequencies in the 
band, might show how
    this offset is related to various correlator signal frequencies, etc.